1. Field of the Invention
The present invention generally relates to a switching circuit for an electrical machine.
2. Description of Related Art
Electric machines, such as, poly-phase claw pole alternators have been readily used in automobile applications. These alternators produce poly-phase AC currents and rectifiers are employed to convert the AC currents into DC currents to charge vehicle batteries and supply electrical power to electric vehicle devices.
In almost all vehicle alternators, P/N junction diodes are used as rectifying elements to convert AC currents into DC currents. The forward voltage drop of these power electronic P/N diodes is around 1 volt. About 8-10% of the total energy provided to an alternator will dissipate as heat in the rectifier. This heat dissipation reduces the alternator efficiency and increases temperature of rectifier and regulator.
Some other semi-conductor devices, such as MOSFET, IGBT, SCR, etc. have lower forward voltage drop when used as rectifying elements. One common feature of these semi-conductor devices is that electric control signals are required to determine the open or close status of these devices. They are controlled switches and abbreviated as switches in this invention. Among these switches power MOSFETs have the best characteristics for alternator applications in today's automobiles with 14-volt electic power system. Diodes are passive rectifying elements and require no control signal, however, MOSFETs are active rectifying elements and often require complex control signals.
Although some rectifiers with MOSFETs as rectifying elements have been developed, the main drawback of MOSFET rectifiers is the cost and complexity of the supporting electronics. Typically, expensive IC circuits are employed to supply voltage signals to control MOSFETs. These IC circuits significantly increase the cost of vehicle alternators. Therefore, almost no mass production automobiles use alternators with MOSFET rectifiers.
The IC control circuit includes a synchronous rectifier controller and some switch gate drivers. The synchronous rectifier controller detects the angle difference between phase current and phase voltage and determines when an individual MOSFET should be in open or closed state. The switch gate drivers, which are controlled by the synchronous rectifier controller, provide voltage signals with proper magnitudes and timing, between gate pins and source pins of the MOSFETs. The MOSFETs, typically, implement a 6-step rectifying procedure for a 3-phase alternator and convert the AC currents from stator phase windings into a DC current.
The IC control circuit and switch gate drivers are sophisticated in that, often voltages of the source pins of up-side MOSFETs are “floating”, therefore, an isolated voltage source is required for every individual up-side MOSFET. The cost of these control and gate driving circuits is quite high for an automobile alternator application.
In view of the above, it is apparent that there exists a need for an improved switching circuit for an electrical machine.